Coastal regions are experiencing a constant increase in population due not only to tourism but also to an aging population from wealthier countries who are able to enjoy retirement in places far from, and more agreeable than, the industrial and commercial towns and cities where they have spent their working life. However, their demand for all the modern comforts they are accustomed to has resulted in coastal resorts having to meet an ever-rising need for a water supply suited to every type of domestic use. This is occurring in every country where tourism, retirement and coastal resorts coincide. Paradoxically, it is where there is an abundance of sea-water that there is also a scarcity of fresh water. Many different methods have been adopted in the search to provide an adequate domestic water-supply, ranging from the modification of river-basins to the construction of devices designed to remove salt from the readily accessible sea water.
There are basically three different methods employed by desalination plants each of which gives rise to various sub-processes.
The method mostly frequently found is that known as “inverse osmosis”. It is based on the use of semi-permeable membranes which allow water, but not salt, to pass through. High-pressurization techniques are required in this process to overcome the osmotic pressure whose natural action is the contrary of that desired in desalination. This system treats part of the water drawn from the sea, returning the remainder, with a corresponding increase in salinity, to the sea. A counter-pressure effect is needed to cleanse the filters. These plants are costly; there are few enterprises producing membranes; maintenance requires much skill and attention. Energy consumption is low.
Another method, namely that of freezing sea-water to obtain salt-free water is based on the displacement of salt ions that occurs when water containing them is frozen. This freezing process must be carried out in layers of geometric progression to allow displacement of the non-frozen water, and at a speed which allows the salt ions to move while the water is still liquid. These plants consume the energy necessary to change sea-water from liquid to solid. Various procedures have been devised to help lower consumption and recycle energy.
Because of its simplicity, the vaporization-condensation method is more commonly used than the freezing process. However, it has the drawback of an unfortunately high consumption of energy since the heat necessary for changing liquid to steam must always be provided. Various systems for reducing this energy consumption have been devised such as lowering pressure so that the liquid boils at a temperature lower than the 100° C. necessary at atmospheric pressure: waterfall vaporization: use of solar energy, etc., but in every case heat has to be provided to convert the material from one state to another. Therefore, the most appropriate way of reducing energy consumption is by recovering heat generated. The chief drawback in systems for recovering energy lies in the manner of carrying out the recovery.
The invention presented here avoids this pitfall by a simple system in which sea-water is used in the condenser's cooling circuit and this consignment of water is then sent to the vaporizer where, as less than 10% of the water evaporates, the volume of water in the cooler can be adjusted for the transference of heat.
Bearing in mind just how abundant sea-water is, the invention is designed to desalinate only a portion of the water it takes from the sea, returning the rest to its source. It is calculated that the system proposed here will allow desalination of 5-7% of the volume of water drawn from the sea; the remainder being returned corresponding increase in salinity. Accordingly, the invention provides a desalination plant using the vaporization-condensation system, at low temperatures, in continuous session, and recycling the heat generated during condensation.
In the present invention, water drawn from the sea and preheated up to 40° C. is used for evaporation (A temperature of 60° C. is considered the upper limit.) Two phenomena occur during evaporation: first, the cooling of the water to 20° C., secondly, the heat difference in the water is instrumental in evaporating a small part of it. The evaporated water is sent to a cooling unit whose mission is to condense it to be free of salt. A static high-pressure fan is responsible for sending the steam from the vaporization area to the condensation area; The invention makes use of the fan's characteristics to create a difference in pressure (equal in magnitude to the pressure of steam at vaporization temperature) between the condensation and the vaporization zones. The process is carried out in the presence of an air-current which acts as the vehicle for transporting the steam. The air is recycled within a closed circuit. The condenser-cooler is refrigerated with sea-water and the outlet of the refrigeration circuit is connected with the entrance to the vaporizer.